Printing lists with commas C++

Question

I know how to do this in other languages, but not C++, which I am forced to use here.

I have a Set of Strings that I'm printing to out in a list, and they need a comma between each one, but not a trailing comma. In java for instance, I would use a stringbuilder and just delete the comma off the end after I've built my string. How do I do it in C++?

auto iter = keywords.begin();
for (iter; iter != keywords.end( ); iter++ )
{

    out << *iter << ", ";
}
out << endl;

I initially tried inserting this block to do it (moving the comma printing here)

if (iter++ != keywords.end())
    out << ", ";
iter--;

I hate when the small things trip me up.

EDIT: Thanks everyone. This is why I post stuff like this here. So many good answers, and tackled in different ways. After a semester of Java and assembly (different classes), having to do a C++ project in 4 days threw me for a loop. Not only did I get my answer, I got a chance to think about the different ways to approach a problem like this. Awesome.

Answer 1

One common approach is to print the first item prior to the loop, and loop only over the remaining items, PRE-printing a comma before each remaining item.

Alternately you should be able to create your own stream that maintains a current state of the line (before endl) and puts commas in the appropriate place.

EDIT: You can also use a middle-tested loop as suggested by T.E.D. It would be something like:

if(!keywords.empty())
{
    auto iter = keywords.begin();
    while(true)
    {
        out << *iter;
        ++iter;
        if(iter == keywords.end())
        {
            break;
        }
        else
        {
            out << ", ";
        }
    }
}

I mentioned the "print first item before loop" method first because it keeps the loop body really simple, but any of the approaches work fine.

Answer 2

I think this should work

while (iter != keywords.end( ))
{

    out << *iter;
    iter++ ;
    if (iter != keywords.end( )) out << ", ";
}

Answer 3

There is a little problem with the ++ operator you are using.

You can try:

if (++iter != keywords.end())
    out << ", ";
iter--;

This way, ++ will be evaluated before compare the iterator with keywords.end().

Answer 4

Something like this?

while (iter != keywords.end())
{
 out << *iter;
 iter++;
 if (iter != keywords.end()) cout << ", ";
}

Answer 5

Use an infix_iterator:

// infix_iterator.h 
// 
// Lifted from Jerry Coffin's 's prefix_ostream_iterator 
#if !defined(INFIX_ITERATOR_H_) 
#define  INFIX_ITERATOR_H_ 
#include <ostream> 
#include <iterator> 
template <class T, 
          class charT=char, 
          class traits=std::char_traits<charT> > 
class infix_ostream_iterator : 
    public std::iterator<std::output_iterator_tag,void,void,void,void> 
{ 
    std::basic_ostream<charT,traits> *os; 
    charT const* delimiter; 
    bool first_elem; 
public: 
    typedef charT char_type; 
    typedef traits traits_type; 
    typedef std::basic_ostream<charT,traits> ostream_type; 
    infix_ostream_iterator(ostream_type& s) 
        : os(&s),delimiter(0), first_elem(true) 
    {} 
    infix_ostream_iterator(ostream_type& s, charT const *d) 
        : os(&s),delimiter(d), first_elem(true) 
    {} 
    infix_ostream_iterator<T,charT,traits>& operator=(T const &item) 
    { 
        // Here's the only real change from ostream_iterator: 
        // Normally, the '*os << item;' would come before the 'if'. 
        if (!first_elem && delimiter != 0) 
            *os << delimiter; 
        *os << item; 
        first_elem = false; 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator*() { 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator++() { 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator++(int) { 
        return *this; 
    } 
};     
#endif 

Usage would be something like:

#include "infix_iterator.h"

// ...
std::copy(keywords.begin(), keywords.end(), infix_iterator(out, ","));

Answer 6

Try this:

typedef  std::vector<std::string>   Container;
typedef Container::const_iterator   CIter;
Container   data;

// Now fill the container.


// Now print the container.
// The advantage of this technique is that ther is no extra test during the loop.
// There is only one additional test !test.empty() done at the beginning.
if (!data.empty())
{
    std::cout << data[0];
    for(CIter loop = data.begin() + 1; loop != data.end(); ++loop)
    {
        std::cout << "," << *loop;
    }
}

Answer 7

Because everyone has decided to do this with while loops, I'll give an example with for loops.

for (iter = keywords.begin(); iter != keywords.end(); iter++) {
  if (iter != iter.begin()) cout << ", ";
  cout << *iter;
}

Answer 8

My typical method for doing separators (in any language) is to use a mid-tested loop. The C++ code would be:

for (;;iter++) {
   std::cout << *iter;
   if (iter == keywords.end()) break;
   std::cout << ",";
}

(note: An extra if check is needed prior to the loop if keywords may be empty)

Most of the other solutions shown end up doing an entire extra test every loop iteration. You are doing I/O, so the time taken by that isn't a huge problem, but it offends my sensibilities.

Answer 9

Could be like so..

bool bFirst = true;
for (auto curr = keywords.begin();  curr != keywords.end(); ++it)
{
   std::cout << bFirst ? "" : ", " << *curr;
   bFirst = false;
}

Answer 10

You can use a do loop, rewrite the loop condition for the first iteration, and use the short-circuit && operator and the fact that a valid stream is true.

auto iter = keywords.begin();
if ( ! keywords.empty() ) do {
    out << * iter;
} while ( ++ iter != keywords.end() && out << ", " );
out << endl;

Answer 11

Assuming a vaguely normal output stream, so that writing an empty string to it does indeed do nothing:

const char *padding = "";
for (auto iter = keywords.begin(); iter != keywords.end(); ++iter) {
    out << padding << *iter;
    padding = ", "
}

Answer 12

I would go with something like this, an easy solution and should work for all iterators.

int maxele = maxele = v.size() - 1;
for ( cur = v.begin() , i = 0; i < maxele ; ++i)
{
    std::cout << *cur++ << " , ";
}
if ( maxele >= 0 )
{
  std::cout << *cur << std::endl;
}